1. Field of the Invention
The present invention relates to an antireflection film which is provided on a surface of an optical filter, a lens, or the like, a lens, and an imaging device.
2. Description of the Related Art
An optical device in which an optical substrate such as a lens or various optical filters is used, for example, a digital camera, an image scanner, or a display device such as a liquid crystal display device or a projector has been widely used. A shape or an optical operation of the optical substrate varies depending on the intended use and the like. In either case, in general, an antireflection film is provided on a surface of the optical substrate. This configuration is to prevent the light use efficiency from being decreased due to loss caused by surface reflection of a lens or various optical filters.
As the antireflection film, for example, a multi-layer film including a plurality of thin films having different refractive indices is known (JP2002-156507A). In the antireflection film, for example, a combination of layers, refractive indices of layers, the number of layers, or the laminating order vary depending on a wavelength range to be used. For example, in a case where two or more materials are used, a high refractive index film, which is formed of a high refractive index material, and a low refractive index film, which is formed of a low refractive index material having a lower refractive index than the high refractive index film, are alternately laminated.
In addition, an antireflection film in which a diamond-like carbon (DLC) film as a protective layer is provided on the top layer is known (JP2010-181514A and JP2008-268281A).
In a case where the combination of the high refractive index film and the low refractive index film is used as described in JP2002-156507A, the low refractive index film is disposed on the outermost layer (first layer on the air side) of the antireflection film in order to reduce Fresnel reflection. As the low refractive index film, a metal fluoride film having a refractive index of about 1.2 at a wavelength of 10.5 μm is used in many cases. However, the metal fluoride film has a low hardness and thus has a problem in that it is easily damaged.
On the other hand, in a case where a DLC film as a protective film is provided on the top layer as described in JP2010-181514A and JP2008-268281A, the improvement of abrasion resistance can be realized by the DLC film. However, the refractive index of the DLC film is about 2, and the Fresnel reflection intensity thereof is about four times that of the metal fluoride film. Therefore, even in a case where a DLC film is disposed on the top layer simply in order to improve abrasion resistance, there is a problem in that desired reflection properties cannot be obtained.
In addition, in a case where a DLC film is provided as a protective layer as described in JP2010-181514A and JP2008-268281A, it is necessary that an interlayer is provided between the metal fluoride film and the DLC film in order to improve adhesiveness. Therefore, it is necessary to provide an interlayer or to form a low refractive index film using an interlayer. Thus, the degree of freedom in the design of an antireflection film deteriorates, and the optimization is difficult to perform. Further, in a case where a DLC film is provided simply as a high refractive index film, there is a problem in that the DLC film may crack after the formation.